(note: you can combine multiple monitors into one port via the tripplehead2go.)
A single 7900gs can power 2 sperate display signals for 2d apps, You could, although not many people would bother, run as many monitors as you want off of one card by spliting the signal to multiple monitors. However when you do this all of the monitors on one port would display the same image.
When you lanch a 3d app it will run the game on all of the monitors on one of the ports; be it one monitor, a bunch of split signal monitors displaying the same image, or 3 monitors signals turned into one monitor signal making it appear as one large monitor via the tipplehead2go.
If you had two cards you would have 4 sperate ports to use but again when you fire up a 3d app it will only run on one port. You could use the other ports to display say a movie or a text document while you play a 3d game however.
With a tripplehead2go you can run 4 monitors each displaying a different image on one 7900gs, with 3 running on the tripplehead2go on port1 and a seperate monitor running on port2. Your computer will think that the 3 monitors on the tripplehead2go are one monitor but it has 3x the horizontal resolution as one would have. When you fire up say flight simulator X you can run the game on your one large "monitor" that is really the 3 monitors combined into one input all on one port. Also while you play games on your large "monitor" you could watch a movie or something on the 4th monitor.